In the article by D. W. Smith entitled "Coherent Fiber-optic Communications" which was published in FOCUS/Electro-Optics for November 1985 at pages 92 et seq., there is disclosed an optical heterodyne or homodyne receiver wherein the received polarized light beam is combined with a second polarized light beam from a local oscillator; the resultant combined light beam is converted by an optical-to-electric transducer into an electric signal which subject to further processing.
In such an optical receiver, it is desirable for the polarization states of the received light beam and the light beam produced by the local oscillator to match as well as possible. This can be accomplished, for example, by using a polarization-holding fiber for the transmission path to the receiver and connecting it to the receiver in such a way that the polarization directions of the received light beam and the light beam from the local oscillator have the desired relationship to each other. However, polarization-holding optical fibers have certain disadvantages; for example, they have relatively high loss.
A polarization control system is disclosed in published German patent application DE-OS 31 50 697, in which control signals are derived which cause the polarization of the light beam to oscillate to a negligible extent about a predetermined value by decoupling that portion of the light beam which does not have the predetermined polarization direction and feeding it an opto-electric transducer which is followed by mixers and a low pass filter.